Speed control systems for induction motors typically include speed sensors. Due to the increased cost associated with speed sensors, industrial motors are often not equipped with speed sensors. However, knowing the speed or being able to use a signal to control speed of industrial motors is useful.
One approach to sensorless estimation of motor speed has been to look for the rotor bar pass frequency (BPF) in the motor current. BPF is the product of the motor""s rotor shaft speed (MS) and the number of rotor bars (N) in the rotor cage. Unfortunately many combinations of rotor poles and slotting result in very small amplitude of the BPF such that the relevant signal becomes lost in the welter of noise and other harmonics. In motors where the BPF is more readily detectable, long sampling times and complex computations are required to precisely determine bar pass frequency. The time periods typically range from a few seconds to about ten seconds. Thus, these techniques are only applicable to slow speed control systems.
It would be desirable to provide a method of sensorless speed sensing by the analysis of motor current that is accomplished without requiring long sampling times and complex computation.
Briefly, in accordance with one embodiment of the present invention, a method comprises: sampling a current waveform of an induction machine to obtain sampled points; characterizing the sampled current over a selected first time window to obtain characterized points; subtracting the characterized points from respective sampled points to obtain a residual signal comprising residual points; obtaining a frequency spectrum representative of the residual signal over a selected second time window; and identifying a rotor bar pass frequency from the frequency spectrum.
In accordance with another embodiment of the present invention, a system comprises a computer configured for: sampling a current waveform of an induction machine to obtain sampled points; characterizing the sampled current over a selected first time window to obtain characterized points; subtracting the characterized points from respective sampled points to obtain a residual signal comprising residual points; obtaining a frequency spectrum representative of the residual signal over a selected second time window; and identifying a rotor bar pass frequency from the frequency spectrum.
In accordance with another embodiment of the present invention, a computer-readable medium stores computer instructions for instructing a computer. The computer instructions comprise: sampling a current waveform of an induction machine to obtain sampled points; characterizing the sampled current over a selected first time window to obtain characterized points; subtracting the characterized points from respective sampled points to obtain a residual signal comprising residual points; obtaining a frequency spectrum representative of the residual signal over a selected second time window; and identifying a rotor bar pass frequency from the frequency spectrum.